Whether listening to an MP3 player while traveling, or to a hi-fi stereo system at home, consumers are increasingly choosing the in-ear earphone for their listening pleasure. Although these acoustic devices have a relatively low profile, which provides convenience, they are also able to provide good sound quality. An in-ear earphone, also referred to as an earbud, is designed to be inserted partially into an ear canal so as to create an airtight seal against the inner surface of the canal. This provides the wearer with good acoustic isolation against external sounds. However, if the wearer were to ride up in an elevator while listening to the earphone, the resulting sudden drop in outside (barometric) pressure may cause the in-ear phone to stop playing. This may be explained as follows.
Consider a typical in-ear earphone that has a receiver case in which a motor is connected to a diaphragm. Note the reference to “receiver” here, which is a type of driver that is designed to be small enough and have the appropriate sound pressure output levels for use in an earphone. An electrical audio signal that is applied to the motor causes the diaphragm to move and thereby create sound pressure, which is directed out of a main sound output port of the earphone (and into the user's ear). The diaphragm has one side (inside the receiver case) that is open to the main sound port, and another that seals the receiver case airtight. When the barometric pressure drops, the air pressure inside the receiver case against the side of the diaphragm that is open to the sound port will also drop, due to “gasket leakage” past the seal made with the ear canal. However the pressure on the sealed side of the diaphragm does not equalize as quickly, thereby causing the diaphragm to “stick” so that no sound is being produced. To avoid this undesirable effect, the diaphragm is manufactured with a calibrated airflow hole, also referred to as a barometric vent hole. The hole is sized to allow the pressures on both sides of the diaphragm to equalize quickly, so as to reduce the likelihood that the diaphragm will be held stuck in the event of a sudden barometric pressure change. But the hole will adversely affect the acoustic response of the receiver and cause the sound output capability to drop somewhat. On balance however, the performance reduction caused by the hole has been tolerated given its beneficial effect on the overall practicality of the earphone.